This invention relates to a phase shifting mask and an exposure method in which the phase shifting mask is used, and more particularly to a phase shifting mask which can be used with various pattern formation techniques and allows formation of a resist pattern on a wafer with a high yield upon production of, for example, semiconductor devices.
In semiconductor devices, the working dimensions of patterns progressively become finer year by year, and such fine patterns cannot be resolved sufficiently with conventional photo-masks which are constituted only from a light shielding area and light transmission areas. Therefore, a phase shifting mask with which a fine pattern which cannot be resolved with any conventional mask can be resolved is spotlighted in place of conventional photo-masks. The phase shifting mask has an ordinary first light transmission area for formation of a pattern, and an auxiliary second light transmission area which provides transmission light having a phase different by 180 degrees from that of transmission light through the first light transmission area.
When it is tried to use a phase shifting mask for formation of isolated patterns as represented by, for example, contact holes, several phase shifting masks including a phase shifting mask of the outrigger type in which an auxiliary pattern which provides transmission light having a phase relatively different by 180 degrees from that of transmission light through a main pattern is disposed in the proximity of the main pattern, another phase shifting mask of the rim type in which a similar auxiliary pattern is disposed adjacent a main pattern and a further phase shifting mask of the attenuated type in which a light semi-shielding portion which provides semi-transmission light having a phase relatively different by 180 degrees from that of transmission light through a main pattern is provided are principally employed. Those conventional phase shifting masks can be applied where patterns are not disposed comparatively densely, and provide remarkable enhancements in resolution and depth of focus comparing with conventional masks.
The conventional phase shifting masks of the types described above are disadvantageous in the following points when it is tried to form a large number of rectangular isolated patterns arranged comparatively densely.
In particular, the phase shifting masks of the outrigger type and the rim type are disadvantageous in that, since the width of a masking area necessary for formation of a single rectangular pattern is greater than twice the width of the pattern on the mask, it is difficult to reduce the distance (hereinafter referred to as pattern distance) between two adjacent patterns on a wafer and particularly it is impossible in principle to dispose rectangular patterns at such a small distance with which the pattern width and the distance between the centers of adjacent patterns in the light transmission area exhibit a ratio of 1:2.
On the other hand, although the phase shifting mask of the attenuated type allows an arrangement of patterns in which the pattern width and the distance between the centers of adjacent patterns in the light transmission area exhibit a ratio of 1:2, it is disadvantageous in that, since side lobe light of a pattern upon operation to transfer of a masking pattern onto a wafer is overlapped with main lobe light of an adjacent pattern and besides the side lobe light and the main lobe light are different by 180 degrees in phase from each other, the side lobe light remarkably reduces the original effect of a phase shift and formation of a good pattern is disturbed.
It is to be noted that, also with conventional masks which do not employ phase shifting, since side lobe light of a pattern is overlapped with main lobe light of an adjacent pattern and besides the side lobe light and the main lobe light are relatively different by 180 degrees in phase from each other, the side lobe light disturbs formation of a good pattern similarly as described above. Therefore, conventionally it is particularly difficult to dispose fine rectangular patterns so that the ratio between the pattern width and the distance between the centers of adjacent patterns in the light transmission area may be substantially 1:2.